title: Risk of Guillain-Barré syndrome after seasonal influenza vaccination and influenza health-care encounters: a self-controlled study.
abstract: BACKGROUND
  The possible risk of Guillain-Barré syndrome from influenza vaccines remains a potential obstacle to achieving high vaccination coverage. However, influenza infection might also be associated with Guillain-Barré syndrome. We aimed to assess the risk of Guillain-Barré syndrome after seasonal influenza vaccination and after influenza-coded health-care encounters.
  
  
  METHODS
  We used the self-controlled risk interval design and linked universal health-care system databases from Ontario, Canada, with data obtained between 1993 and 2011. We used physician billing claims for influenza vaccination and influenza-coded health-care encounters to ascertain exposures. Using fixed-effects conditional Poisson regression, we estimated the relative incidence of hospitalisation for primary-coded Guillain-Barré syndrome during the risk interval compared with the control interval.
  
  
  RESULTS
  We identified 2831 incident admissions for Guillain-Barré syndrome; 330 received an influenza vaccine and 109 had an influenza-coded health-care encounter within 42 weeks before hospitalisation. The risk of Guillain-Barré syndrome within 6 weeks of vaccination was 52% higher than in the control interval of 9-42 weeks (relative incidence 1·52; 95% CI 1·17-1·99), with the greatest risk during weeks 2-4 after vaccination. The risk of Guillain-Barré syndrome within 6 weeks of an influenza-coded health-care encounter was greater than for vaccination (15·81; 10·28-24·32). The attributable risks were 1·03 Guillain-Barré syndrome admissions per million vaccinations, compared with 17·2 Guillain-Barré syndrome admissions per million influenza-coded health-care encounters.
  
  
  CONCLUSIONS
  The relative and attributable risks of Guillain-Barré syndrome after seasonal influenza vaccination are lower than those after influenza illness. Patients considering immunisation should be fully informed of the risks of Guillain-Barré syndrome from both influenza vaccines and influenza illness.
  
  
  BACKGROUND
  Canadian Institutes of Health Research.
authors.affiliation: Institute for Clinical Evaluative Sciences, Toronto, Canada. jeff.kwong@utoronto.ca
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Kwong
authors.forename: Priya P
authors.initials: PP
authors.lastname: Vasa
authors.forename: Michael A
authors.initials: MA
authors.lastname: Campitelli
authors.forename: Steven
authors.initials: S
authors.lastname: Hawken
authors.forename: Kumanan
authors.initials: K
authors.lastname: Wilson
authors.forename: Laura C
authors.initials: LC
authors.lastname: Rosella
authors.forename: Therese A
authors.initials: TA
authors.lastname: Stukel
authors.forename: Natasha S
authors.initials: NS
authors.lastname: Crowcroft
authors.forename: Allison J
authors.initials: AJ
authors.lastname: McGeer
authors.forename: Lorne
authors.initials: L
authors.lastname: Zinman
authors.forename: Shelley L
authors.initials: SL
authors.lastname: Deeks
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002170:Canada / Q000453:epidemiology; D005260:Female; D020275:Guillain-Barre Syndrome / Q000139:chemically induced* / Q000453:epidemiology; D006760:Hospitalization; D006801:Humans; D015994:Incidence; D007252:Influenza Vaccines / Q000009:adverse effects*; D007251:Influenza, Human / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012621:Seasons; D014611:Vaccination / Q000009:adverse effects* / Q000706:statistics & numerical data; D015164:Vaccines, Inactivated / Q000009:adverse effects; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines; D015164:Vaccines, Inactivated